The invention disclosed and claimed herein generally pertains to housing apparatus which includes a window or lens which must be transmissive to radiation of selected frequency, wherein it is necessary to regulate the temperature of the window to prevent the formation of ice or water deposits thereupon. More particularly, the invention pertains to such housing apparatus wherein the window comprises a spherical segment and is heated by a bearing gasket, the bearing gasket being capable of withstanding extremely high temperatures and compression forces. Even more particularly, the invention pertains to such apparatus for housing optical or infrared sensing equipment in a marine environment.
It has been found that a window having the shape or configuration of a spherical segment, such as a hyperhemisphere, may be very usefully employed in a submarine periscope housing for protecting optical or infrared image sensing equipment. By employing a spherical segment window in such a housing, various advantages have been realized, such as improved viewing capability and reduced resistance to motion through water. A periscopic housing which employs a hyperhemispherical window has been successfully designed, fabricated and tested, and is disclosed in a Patent Application of Jerry D. Stachiw and George M. Horn for a "Pressure Resistant Housing," filed 23 Sept. 1977, Ser. No. 836,255, a communication indicating the allowance of the application having been mailed from the Patent and Trademark Office on 29 Dec. 1978.
In a housing for protecting infrared viewing or sensing equipment, the window for the housing is usually formed for germanium, which is highly transmissive to radiation in the infrared frequency range. If such a housing is to be used in a frigid marine environment, such as the Arctic Ocean, the problem of ice formation on the external surface of the window is encountered, especially when the window is near the seawater-air interface, and is repeatedly subjected to exposure to Arctic winds and submersions in near freezing seawater. The problem of ice formation is very significant, since water and ice coatings are impenetrable barriers to radiation of infrared frequencies.
Current techniques for heating germanium windows employed in high altitude applications, such as in aircraft infrared sensory equipment, use the resistivity of the germanium by applying an electric potential or voltage directly to the window. However, in a seawater environment, the direct application of electric potential to a germanium window would result in electrolysis of the window, whereby the window would be severly corroded. Consequently, infrared sensing equipment for marine applications is currently restricted to operation in temperature waters. As far as is known, no infrared viewing or sensing equipment is available for operating in a frigid or Arctic marine environment.
In the above-referenced Pressure Resistant Housing, a germanium or glass hyperhemispherical window, provided with a seating surface, is sealably joined to the bearing surface of a mounting or base. A germanium window is used when the housing contains infrared sensing equipment, and a glass window is used when the housing contains equipment for sensing optical images. A gasket of elastic material is positioned between the seating and bearing surfaces in order to resist wear therebetween, particularly lateral wear. All of the forces required to support the window and to resist external pressure thereupon are provided by the mounting and must be transferred through the gasket. If the housing is employed as part of a submarine periscope system, external pressure on the window may exceed several thousand psi when the submarine is submerged, whereupon the gasket is subjected to extreme compressive force.
It has been considered that in the above type of housing, a germanium or glass window could be maintained at a temperature above the freezing point of water by enclosing an electric heating element in the gasket of the housing. The gasket would be heated by the electric element, and heat transferred to the window therefrom by the process of heat conduction. However, it would be necessary to structure such a heating element-bearing gasket combination so that the heating element would not be crushed or damaged by the extreme compressive forces upon the gasket, or by lateral motion of the window seating surface with respect to the mounting bearing surface. Also, it is anticipated that in order to maintain the external surface of a germanium or glass hyperhemispherical window at a temperature of 30.degree. F.-40.degree. F. in a frigid environment, it could be necessary to heat the gasket to a temperature in excess of 200.degree. F. Repeated heating of the gasket to such extent could seriously reduce or destroy its elastic properties. This could be very serious if the gasket was used in a housing for a submarine periscope system. In such a system, in addition to resisting wear between the mounting and the window, an elastic gasket is required to provide compensation for the very different flexure rates of the mounting and window as they are subjected to varying amounts of hydrostatic pressure. The different flexure rates result from the construction of the mounting from a metal such as titanium, whereas the window is formed from a brittle material such as germanium or glass.